Under the trademark practice in Japan, a Power of Attorney (PoA) is not needed to be filed to the Japan Patent Office (JPO) for filing a new trademark application.
However, a PoA must be filed when you appoint a new representative against the JPO after filing a trademark application.
Especially, I would like to emphasize that you need to prepare a PoA duly signed by Applicant to respond to Office Action issued for a Japanese trademark application based on an international registration (by the Madrid System) through a new representative domiciled in Japan.
The JPO always requires us to submit an original PoA. So please keep it in your mind that you need enough time to prepare the PoA before a deadline for response to Office Action.